Sound recording



June 1o, 1930.

R. c. MATHES 1,763,169

soUND RECORDING Filed sept. 25, 192e 3 snetsfsheet 1 f7? )0 4 /V 3 /5 /4 WM-irma# June 1o, 1930.

R. C. MATHES SOUND RECORDING Filed Sept. 23, 1926 s sheets-sheet 2 June 10,1930. R Q MATHES 1,763,169

SOUND RECORDING Filed Sept. 23. 1926 3 Sheets-Sheet 3 59 line and aline balancing network by ahybrid volume range limited to a telephone tran- 199 Patented June' 1o, 1930 lI JNrrrzu sra'rss j PArsN ori-'ic ROBERT C. MATHES, OF WYOMING, NEW, JERSEY, ASSIGNOR TO BELL TELEPHONE v LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK SOUND nEcoRnrNe Application filed September 23, 1926.v Serial No. 137,274.

This invention relates to systems forrecording variations, as for example, variations corresponding` to sound, and aims to avoid excessive changes in the amplitude of the variations applied to the recorder in the system. y

As a specific example of applications of the invention, there are described hereinafter systems for recording incoming andoutgoing speech at a telephone station, such as a subscribers or operators telephone set, the re corder being of phonographic or any other desired type. One of the problems to be met in such systems is that of making the minimum power input into the recorder suihcient to yield a satisfactory record, and preventing the maximum power input to the recorder from unduly overloading the recorder. The input volume encountered may be large due to a loud voice at the local transmitter, and may be low due to currents originated by a weak voice at the distant end 'of a long toll connection. For reducing the tendency of the current generated in the local transmitter to overload the recorder, the recorder may be connected in conjugate or anti-side tone relation to the local transmitter, the adjustment of the circuit being made sufficiently different from that required for the condition of conj ugacy to give the desired amount of side tone in the recorder. Egerton Patent 1,548,028, August 4, 1925, discloses an anti-'side tone type of substation circuitor telephone set, in which a recorder has the place of the usual receiver and in which the receiver is in series with the line in order that the side tone produced in the receiver may cause the talking' subscriber to soften his tones and thereby lead and consequently increase the energy level at the input of the recorder. ment of the circuit is sufliciently different from that required for conjugacy of the transmitter and the recorder so that the transmitter can operate therecorder.

In specific embodiments of the instant invention shown in the drawing, a sound recorder and a telephone set which includes a transmitter and a receiver are coupled to a The adjustl corder or the telephone set can occupy either the bridged circuit or the. series circuit position in this bi-conjugate system, or in other words, can be Veffectively connected to either the bridge points or the series terminals of the hybrid coil. The telephone set can be of any desired type. For example, the transmitter can be either effectively in series or effectively in shunt With the receiver, with respect to the circuit faced by the telephone set, or can haveany desired relation to the receiver as regards transmission of side tone from the transmitter to the receiver. Enough unbalance is used in the hybrid coil connection of the elements of the bi-conjugate system to give any desired amount of side tone in the recorder. j

Preferably, an electric space discharge ampliler having a power limiting characterpling a sound recorder and a telephone set which includes a transmitter and receiver to a line, with the recorder and the telephone set in conjugate relation to each other, the adjustment of the circuit being suiiiciently dif-l ferent from that required for the condition of conjugacy to give the desired amount of side tone in the recorder.

Another feature of the invention is a sound recorder having a volume range limiting device in its input circuit.

A somewhat different aspect of the invention is the conjugate relationship of a sound recorder which has in its input circuit a mitter, the adjustment of the circuit being `such that the recorder receives the desired amount of side tone from the transmitter.

Other objects and aspects of the invention will-be apparent from the following description and claims.

y Fig. 1 of the drawings is a circuit diagram for facilitating explanation of the circuits of Figs. 2 to 7 Figs. 2 to 7 show six different forms of the invention; and Fig. 8 is a curve for facilitating explanation of the operation of an amplifier preferably employed in the circuits of Figs. 2 to 7. A

In Fig. 1, a telephone line 10 anda balancing impedance network 11 therefor are connected by a hybrid coil 12. The hybrid coil comprises a line winding 13, a network winding 14, and a series winding or arm 15. These windings are inductively related in the manner usual for hybrid coils,l as employed in telephone repeater circuits, for example. Terminals 1 and 2 of the series arm 15 are the series impedance terminals of the hybrid coil. Terminals 3 and 4 are the bridge points or bridge terminals of the hybrid coil.

Each of Figs. 2 to 7 shows a subscribers telephone set, including a transmitter 20and a receiver 21, a sound or voice recorder 22 which preferably has an electric space discharge amplifier 23 in its input circuit, a telephone line 10, an impedance network 11 for balancing the impedance of line 10, and a hybrid coil connecting the telephone set and the recorder amplifier to the line 10 in conjugate relation to each other. As pointed out in more detail hereinafter, certain of these figures show the telephone set effectively connected to the bridge points `of the hybridcoil, and the recorder amplifier connected to the series impedance terminals of the hybrid coil, whereas others of the figures show the telephone set effectively connected to the series impedance terminals of the hybrid coil and the Yrecorder amplifier connected to the bridge points of the hybrid coil.

In Fig. 2 the telephone set is connected to the bridge points 3 and 4 of hybrid coil 12,`

and the recorder and amplifier are connected to the series impedance terminals 1 and 2 of the hybrid coil. The telephone set is shown in this figure as a series set, the transmitter 20 and receiver 21 being effectively in series with each other with respect to the line.

In Vthe operation of the subscribers station shown in Fig. 2 ringing current from line 10 passes through ringer 25 and large condenser 26. When the receiver operated switch 27 is closed direct current from line 10 energizes transmitter 2O over a circuit extending from the upper conductor of line 10 through transmitter 20, the lower portion of induction coil 28, receiver 21, and winding 13 of hybrid coil 12, and receiver operated switch 27, to the lower conductor of line 10. Condenser 26 prevents direct current from being by-passed around transmitter 20 through the upper portion of induction coil 28. Operation of transmitter 20 causes voice currents to flow through condenser 26 and the J ceiver 21', over a circuit extending from lower conductor of line 10, through switch 27, winding 13, receiver 21, lower portion of coil 28 and upper portion of coil 28 and condenser 26 in parallel with transmitter 20, to the upper conductor of line l10. The flow of this current through winding 13 induces in winding 15 an electromotive force for causing operation of amplifier 23 and recorder 22. If desired,pnetwork 11 may include a condenser preventing fiow of direct current through the network, or the upper network terminal may be moved down to the lower terminal of condenser 26, for the same purpose. In some cases induction coil 28 may be omitted and the transmitter 20 connected directly yto the receiver 21.

Curve A 30 of Fig. 8 is plotted between transmission levels at the input of the amplifier 23 as abscissae and transmission levels at the output fof the amplifier as ordinates, the transmission level in each case being expressed in transmission units. The amplifier has a limited load capacity and is adjusted to bring the minimum speech levels at the input of the recorder up to a volume level just suitable for recording. For volume levels more than, say, 25 or 30 t. u. above the minimum level, the amplifier is overloaded, so that the loudest speech levels applied to the input are not recorded as more than about 25.t. u. greater in volume than the weakest levels. The amplifier thus functions as a volume range'limiter. The point p of the curve at which the slope of the curve begins to decrease noticeably .is controlled by the direct current voltages applied to the amplifier. The output level corresponding to the point Z of the curve, which marks the beginning of the horizontal portion of the curve, indicates approximately the amplier input and output levels at which noticeable overloading of the recorder begins.

In Fig. 3 the amplifier and recorder are connected to the bridge points 3 and 4 of hybrid coil 12 and the telephone set is connected to the series impedance terminals 1 and 2 of the hybrid coil. The telephone set mitter 20and receiver 21' being e e'ctively in series with each other with respect to' the line.

In the opcrationiof the subscribers station` shown in Fig. 3' ringing current from line 10 passes through ringer 25 and large con-- denser 40. When the receiver operated switch 27 yis closed direct current from line.

10 energizes transmitter 2O over a circuit extending from the upper conductor of line 10, through a low resistance. primary winding of an input transformer (not shown) for amplifier 23, winding 13 of hybrid coil- 12, switch 27, winding 15 of hybridcoil 12, right hand portion of induction coil 28, transmitter 20, and receiver 21, to the lower conductor of line 10. Operation of transmitter 20 causes voice currents to. flow vthrough condenser 26 and the left hand portion ofl in amount of side tone for operating amplifier 23 and recorder'22.

Voice currents transmitted to the substation over line 10 operate the amplifier and recorder over a circuit extending from the lower conductor of' line 10, through condenser 4 0, switch 27 winding 13, the low resistance primary winding of the input transformer (not shown) for amplifier 23, to the upper conductor of line 10. This current in winding 13 induces in' winding 15 an electromotive force which operates the receiver' 21 over a circuit extending from terminal 1, through switch 27, condenser 40, receiver 21, condenser 26 and left hand portion of coil 28 in parallel with transmitter 20. and right hand portion of coil 28 to terminal 2.

In Fig. 4 the telephone set is connected to the bridge points 3 and 4 of hybrid coil 12 and the recorder and amplifier are connected to the series impedance termina-ls 1 and 2 of the hybrid coil. The telephone set is shown in this figure as a standard series set, the transmitter 20 and receiver 21 being eectively in series with each other with respect to the line.

In the operation of the subscribers station shown in Fig. 4 ringing current from line 10 passes through ringer 25, and large condenser 35. When the receiver operated switch 27 is c losed direct current from line 10 energizes transmitter 2O over a circuit extending from lower conductor of line 10,

vthrough switch 27 winding 13 of hybrid coil 12, right hand winding of induction coil 36' and transmitter 20, to upper conductor of line 10.

i The operation of transmitter 2() causes voice current to flowthrough condenser 35, receiver 21 Iand left hand winding of induction coil 36, andalso through right hand winding of induction coil 36 and thence through winding13 switch 27 and line 10 in parallel with winding 14 and network 11. Sufficient unbalance is used inthe hybrid coil connection between the telephone set and the amplifier 23 so that this current throu h windings 13'and 14 will operate the amp ivfier and the recorder. Voice currents coming into the substation over line 10 pass through the circuits traced above for the transmitter through left hand winding of induction coil 36, receiver 21 and condenser 35. The voiceA current from the line in passing through winding 13 of hybrid coil 12 induces in winding 15 an electromotive force for causing the operation of amplifier 23 and recorder 22.

In Fig.. 5 the hybrid coil has no series winding in addition to the line winding and network winding 14, and the series impedance terminals l and 2 of the hybrid coil are at the junction points of the line and network.l windings with the line and the network, re-

spectively. The telephone setis a standardV series subset connected to the series input terminals 1 and 2, the transmitter and the receiver being effectively in series with each other with respect to the lines. The amplifier and the recorder are connected to the bridge points 3 and 4 of the bi-conjugate circuit. Ringing currents from line 10 pass through ringer 25 and large condenser 40. Vhen switch 27 is closed, direct current for the transmitter is supplied throughi a circuit extending from the lower vconductor of line 10, through switch 27, transmitter 20, right hand winding of induction coil 36, winding 14, and a low resistance primary winding of an input transformer (not shown) for amplifier 23, to the upper conductor of line 10. 0peration of the transmitter 20 causes voice current to flow through the left hand winding of coil 36, receiver 21, condenser 35 and switch 27, and also causes current to flow through right hand winding of coil 36, coils 14 and 13, in parallel with network 11 and line 10, and through switch 27. Suflicient unbalance is used in the conjugate connection between the telephone set and the amplifier 23 to enablethe transmitter to operate amplifier and recorder. Voice current received at the sub- Y station energizes receiver 21 over a circuit exprimary winding of the input transformer (not shown) for amplifier 23, to the upper conductor of line 10. This current in winding 13 causes current to flow through windparallel with receiver ing 14, right hand winding of coil 36 and the circuits including the left hand winding of coil 36, receiver 21 and condenser 35, and transmitter 20 and switch 27, through condenser 40.

In Fig. 6 the telephone set is a -shunt 'sub-set forming the series impedance or the loi-con- 'ugate circuit, the amplifier and the recorder eing the bridged impedance. As in the preceding figures the windings 13, 14 and 15 are all on one core, windings 13 and 14 being the line and network windings respectively of the hybrid coil and winding 15 being the series winding of the bybrid coil. Winding 15 forms an auto-transformer winding for connecting transmitter 2O and receiver 21, the receiver being connected across the entire winding 15, whereas the transmitter is in effect connected across the horizontal portion of winding 15, since condenser 40 is large. Ringing current from line 10 passes through ringer 25 and condenser 40. Direct current for energizing transmitter 20 is supplied from line 10 over a circuit extendingfrom the upper conductor of line 10, through the horizontal portion of winding 15, 1n

21 and the vertical portion of winding 15. Transmitter 20, switch 27, the low resistance primary winding of the input transformer (not shown) forthe amplifier 23, and winding 13 to the lower conductor of line 10. Operation of transformer 20 causes voice current to flow through conductor 40 and the horizontal portion of wind ing 15, and therefore, causes current to flow lthrough the vertical portion of winding 15 and receiver 21, to energize the receiver 21. The current in winding 15 induces electromotive` force in windings 13 and-14, causing a current to flow through windings 13 and 14, network 11, switch 27, and condenser 40 to line 10. Suicient unbalance is used in the hybrid coil connection between the telephone set and the amplifier and the recorder to enable the transmitter to operate the amplifier and recorder. Voice current coming into the substation over line 10 passes through a cirycuit extending from lower conductor of line 10, through winding 1.3, primary winding of the input transformer (not shown) for amplifier 13, switch 27, and condenser 40 to upper conductor of line 10. This current flowing in winding 13 induces in winding 15 an electromotive forcefor causing operation of the receiver 21.

Fig. 7 shows a modified form of the circuit of Fig. 6. As was the case in Fig. 6 the iso telephone set is a shunt sub-set forming the series impedance of the bi-conjugate circuit, the amplifier 23 and the recorder 22 being the bridged impedance. The windings 13, 14 and 15, as in the preceding figures are all on one core, windings 13 and 14 being the line and network windings respectively of the hybrid coil and winding 15 being the series winding of the hybrid coil. The receiver 21 is connected across a portion of windingA 13 and an equal portion of winding 14, so that the winding 15 and these portions of windings 13 and 14 serve as a transformer for connecting the receiver to the circuit connected to terminals 1 and 2 of the series winding 15 of the hybrid coil. Ringing current from line 1,0 passes through ringer 25 and condenser 40. Direct current for energizing the transmitter 20 is sup lied from line 10 over a circuit extending from the upper conductor of line 10 through winding 15, transmitter 20, switch 27, low resistance primary winding of an input transformer' (not shown) for amplifier 23 and winding 13, to the lower conductor of line 10. Operation of transmitter 10 causes Voice current to flow through winding 15, condenser 40 and switch 27. This current in winding 15 induces an electromotive force in windings 13 and 14. This electromotive force in windings 13 and 14 sends current through a circuit extending from the right hand end of winding 14, through network 11, switch 27, condenser` 40 and line 10 to the left hand end of winding 13. vThe electromotive force in the portion of windings 13 and 14 across which receiver 21 is connected causes operation of the receiver. Sufficient unbalance is used in the hybrid coil connection between the telephone set and the amplifier 23 tc cause operation of the amplifier and recorder. Voice currents coming into the substation over line 10 pass through a circuit extending from the lower conductor of line 10, -through winding 13, the primary winding of input transformer (not shown) for amplifier 23, switch 27 and conductor 40 to thepupper conductor of line 10, thus causing operation of the amplifier23 and the recorder 22. This current in winding 13 also causes operation of receiver 21.

If desired, the amplifier 23 may be omitted from any of the circuits of the Figs. 2 to 7. If the amplifier be omitted the input circuit of the recorder may be made of low impedance to direct current where that is advisable, in order to facilitate the energization of the transmitter.

What is claimed is:

1. A system comprising a telephone set including a transmitter, a line, a recorder adapted for recording electrical variations within a limited amplitude range, means coupling said telephone set and said recorder to said line and in conjugate relationship to each other, said system being so adjusted that said recorder is supplied with a portion of the electrical variations received over said line and from said transmitter, and means com prising a single space discharge amplifying device connected between the recorder and said coupling means, and automatically rerecorder, and having an energy input versus `energy output characteristic with a slope de- `maintain the amplitude range Vof the electrical variations supplied to said recorder within'the recording limits thereof.

2. A system comprising a telephone set including a transmitter, a. line, a network for balancing said line, a recorder for recording electrical variations within a limited amplitude range, said telephone set, vsaid line, said network and said recorder being connected in a Wheatstone bridge circuit so that said line and said network are in conjugate circuit relationship to each other and said telephone set and said recorder are in conjugate relationship to each other, said bridge circuit being so adjusted that a portion of the.

electrical variations received oversaid line and a portion of the electrical variations received from said transmitter are supplied to said recorder, and means connected in the vinput of said recorder and comprising a single electron. discharge amplifying device with apower limiting characteristic, for maintaining the amplitude range of the electrical variations supplied tosaid recorder within the recording limits thereof.

3. A system comprising a recorder adapted for recording electrical variationswitliin a limited amplitude range, a telephone set in cluding a transmitter, an incoming line, a balancing network for said line, a hybrid coil for lconnecting said recorder and said telephone set to said line and to each other, said ybrid coil having a pair of series terminals and a pair of bridge terminals, said recorder being connected effectively to one pair of ter-A minals and said telephone set being connected effectively to the other pair of terminals, said system being adjusted s that said recorder is supplied with the electrical transmitter, and means 'connected between' variations received-over said line and with the electrical variations received from said said hybrid coil and said recorder and comprising a single vacuum tube amplifying de- Vice with a power limiting characteristic determined by the upper recording limit of said recorder and an amplification factor determined by the lower recording limit of said recorder for maintaining the amplitude of the electrical variations supplied to said recorder Within the recording limits thereof.

4. In combination with a recorder for recording electrical variations between definite upper and lower amplitude limits having an input circuit, a source of electrical variations of a wide range of amplitudes; a single space discharge amplifyingdevice fed from said source and feeding said input circuit, foramplifying the vreceived variations of amplitudes below the lower recording limit of said creasing rapidly above 'a point in the region der can satisfactorily record.

5. AA system comprising a line and a subscribers telephone set connected'thereto, a v

transmitter in said set, a recorder of received electrical waves over a limitedamplitude range, in substantially anti-side tone circuit relation to said transmitter, and an am'- plifier comprising a single space discharge device connected between said transmitter and said recorder and adjusted to amplify the received waves of amplitudes below the range of said recorder, and -to overloadv for received waves of amplitudes above the range ofsaid recorder so as to limit the vinput to therecorder to a range over which the recorder can record Y satisfactorily.

- 6. A system according to claim w1, in which said single space discharge amplifying device connected between said coupling means vand said recorder is adjusted to amplify the received variations of low energy level soas to bring them within the recording range of said recorder, and to overload for'received variations of a level higher fthan can be recorded satisfactorily by said recorder.

7. In combinatioma recorder of electrical waves of amplitudes lying only within' a upper limit and a definite lower limit, a source of electrical waves having an` amplitude-range extending beyond said upper limit and said lower limit, and means com-v prising a single space discharging ampli,- fying device for automatically producing and supplying to said recorder electrical waves corresponding to the wavesfrom said source but having amplitudes lying only between said upper limit and said lower limit.

8. In combination, a recorder of electrical waves of amplitudes lying only within a definite upper limit and a definite lower limit,

va source of electrical waves having'an amplitude range extending beyond said upper limit ROBERT o. MATHES.

of the maximum energy input which the re- 

